One of serious problems with vehicles using fossil fuels such as gasoline and diesel is that they cause air pollution. As a method for solving such a problem, a technique of using a chargeable and dischargeable rechargeable battery as a power source for a vehicle has attracted attention. Accordingly, an electric vehicle (EV) capable of being operated only by a battery, a hybrid electric vehicle (HEV) using a battery and a conventional engine, and the like have been developed, and some of them have been commercially available. A nickel metal hydride (Ni-MH) battery is mainly used as a rechargeable battery corresponding to a power source for the EV, the HEV, and the like, and recently, use of a lithium ion battery has also been attempted.
In order to use such a rechargeable battery as the power source for the EV, the HEV, and the like, a rechargeable battery of high output and large capacity is required. To this end, a plurality of small rechargeable batteries (unit batteries) are connected in series and/or in parallel to form a battery module, and a plurality of battery modules are connected in parallel and/or in series to form one middle- or large-sized battery pack to be used.
However, the high power and large capacity rechargeable battery generates a large amount of heat during a charging or discharging process. When the heat of the unit battery generated during the charging or discharging process may not be effectively eliminated, heat accumulation occurs, resulting in deterioration of the unit battery. Further, there is a possibility that, when some of the unit batteries are overheated due to various causes in this process, ignition or explosion may occur. Therefore, a cooling system is indispensable in a middle- or large-sized battery pack of high power and large capacity.
The cooling of the middle- or large-sized battery pack is generally performed by convective heat transfer based on flow of a coolant. For example, a coolant-flow cooling system in which a coolant such as air is flowed between the unit batteries of the battery pack or between the battery modules by a cooling fan, is used. However, in such a system, a temperature deviation between the unit batteries is very large.
In addition to these problems, a vehicle such the EV, the HEV, and the like often operates under harsh conditions. Optimum operating conditions of the unit batteries included in the battery pack may vary depending on various factors, but are generally determined within a specific temperature range. Since the vehicle operates at a low temperature in winter, it is necessary to operate the battery pack in the optimum operating temperature range as described above. In this case, in order to perform a temperature increasing operation instead of the cooling, the operation of the cooling system may be stopped, or the temperature of the coolant (e.g., air) flowing into the cooling system may be increased. However, when the unit cell is in a very low temperature state before that, components of the battery may be damaged, and deterioration thereof may be promoted by a sudden temperature increasing operation.
As an attempt to solve the problems, in order to prevent an explosion risk of the battery due to a rapid temperature rise, a method in which a flame retardant material is included in some of the constituent elements of the battery or in which curing of an electrolyte is performed at a certain temperature or more has been developed. However, these methods may be used as methods for preventing explosion when the battery is in an abnormal operating state, but they are not techniques for suppressing an internal temperature rise, and further, they have a disadvantage that the battery is converted to an irreversible state so that it may no longer be used.
Therefore, there is a high need for a technique that is capable of prolonging cycle-life of the battery by suppressing the temperature rise inside the battery in a normal operating state or by at least lowering a rate of the temperature rise, and that is capable of further improving safety of the battery by suppressing a rapid temperature rise.
On the other hand, a technology of using a material having high latent heat for a specific purpose during phase change thereof is known. For example, a technology that provides a more comfortable environment by applying the material having the high latent heat to a garment and a room interior to induce a gentle temperature change in spite of a sudden external temperature change is known.
In addition, some technologies for applying the above-mentioned characteristics to a battery are also known. For example, in WO 03/061032, a method of putting a battery into a housing containing a material having high latent heat has been proposed in order to prevent a sudden rise in a temperature of a battery as a power source of a medical instrument for implantation to prevent adverse effects on the human body is provided.
However, this method has a limitation in that a temperature difference between individual rechargeable batteries may not be effectively controlled.
Therefore, there is a high need for a technology capable of fundamentally solving these various problems.